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Author Topic: LK-500ZC  (Read 1530 times)
W8JJW
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Posts: 90




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« on: December 19, 2005, 07:21:36 AM »

Hello.
Been using this amp for several years, always through a tuner. Recently the tune-up settings changed drastically on 160-40m. 20m-10m is still the same. The Plate tuning never peaks output, the more capacitance you add, the power keeps going up, until I run up to full scale.
Same when direct connected to a dummy load.

I found a loose ground connection where the Plate choke and the B+ are coupled to ground by a 1000pf disc cap. The Plate choke also had burned sections. I just got a new somewhat larger plate choke and will get that installed. One thing I also noticed is that the plate coupling caps are twice the capacitance indcated in the schematic..2000pf. I also noted that the caps on the 160 and 80m positions on the bandswitch are much higher values than noted. All caps are ceramic disks rated at 12kv. The variable tuning capacitors seem to be the correct capacitance, with no signs of arcing.

I was thinking of replacing the  plate discs with doorknob types, but am unable to find NPO or NP750 type in the values I need to replace what is there now. Any ideas?

The bandswitch is good, no arcing whatsoever.

The caps all test OK. Could it be temp problem with them?

Thanks, John
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W8JI
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« Reply #1 on: December 19, 2005, 05:22:35 PM »

John,

Dentron and Amp Supply was notorious for picking off value parts in tuned circuits, so the schematic might actually show wrong parts that were later corrected.

If the tuning changed and the caps test the same, it can only be:

1. You have something causing less peak plate current, like less drive or low emission. That will reduce plate impedance and require you to use more capacitance in loading. Less drive power means the tank will need more loading capacitance.

2. You could have a shorted turn on the tank inductor.

The plate choke affects tuning setting, not loading setting, and only on 160 meters. By the time the choke affects 80 or 40, 160 would nevere even work and the choke would have to be pretty crisp like burnt toast over much of the winding.

You know of course you can feed a tank backwards, terminating it at the anode end in the calculated load resistance that would equal the tube operating impedance (about 3000 ohms in your case), and use a MFJ259 or similar where the antenna would connect to check operation.  You should be able to adjust tune and load for a 1:1 SWR.

You can do that pretty easy, and then work on the tank COLD and safe.    


 
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I8VYE
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Posts: 16




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« Reply #2 on: December 21, 2005, 01:51:37 AM »

Hello,

  i have a LK450NT (one tube, no tune type), and this is my experience: while making tests on 20 mts ssb on dummy load my daugther started crying very loudly close to the shack. Then my amp showed the same problem you are facing now: low output and max plate cap reached. I tested everything, including cold test of pad caps with a verified capmeter. Nothing found, they tested ok. Then i substituted the pad cap with a doorknob. One year has passed from that failure and everything is still working ok (i'm crossing my fingers!).

I can image is that what happened, in my case, is a strong transient (maybe something, at same time wrong on amp out like a bad contact on the coax) that damaged the pad caps, they are simple high voltage disc caps. The no tune type is full of pad caps, so i substituted many discs with doorknobs i had, fortunately, in the shack, and it works..

From schematics, if i remember, the lk500 has pad caps on 40, 80 and 160. The pads are paralleled each other switching from 160 t0 40 mts. I mean on 40 mts the the total pad cap should be the parallel of the pads connected to the 160, 80 and 40 mts switch positions.

So, having problems on 160, 80 and 40 mts, I'd suspect the 160 mts caps.

All above what happened to my amp, please take it as a simple suggestion.

Best regards and 73's form i8vye/i0 (Joe).

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KA5N
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Posts: 4380




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« Reply #3 on: December 22, 2005, 06:19:28 AM »

To Joe I8VYE

I am enthralled as to what your daughter's loud crying has to do with the tuning problem.

Allen
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W8JJW
Member

Posts: 90




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« Reply #4 on: December 22, 2005, 07:36:24 AM »

Joe, Thanks for your insight. I don't have a daughter though! Smiley

According to my schematic, the caps get switched in, accumulating from 40m to 160m, with the most capactiance on 160m. However, my bandswitch does not do this, so either the bandswitch was replaced sometime during the life of this amp, or Amp Supply made a running change during mfg and did not change the schematic. I have a 4 capacitors (in parallel)on the 160m bandswitch position (about 3450pf). On 80m its about 800pf, then on 40m about 400pf. Another owner of an LK500 indicated he had 4 caps on his 160 position also and that the BS did not accumulate like most other amps do. This amp has all ceramic disks, most are 12kv rated, X5R dialectric.

I would also like to replace the tank caps with doorknobs, but am unable to find the right dialectric in the size I need. My tank caps are 2000pf! (2-1000pf parallel).

For now, I have a new plate choke that I will install with no other changes and see what that gets me.
I plan to verify the input matching is set correctly, etc.

If that doesn't help, then I may start replacing the loading caps one section at a time as they all test OK.

Thanks again for your experience.
73 John W8JJW
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W8JI
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« Reply #5 on: December 22, 2005, 07:47:57 PM »

<According to my schematic, the caps get switched in, accumulating from 40m to 160m, with the most capactiance on 160m. However, my bandswitch does not do this, so either the bandswitch was replaced sometime during the life of this amp, or Amp Supply made a running change during mfg and did not change the schematic.>>

Not unusual to have the schematic disagree with the manufactured wiring.

<< I have a 4 capacitors (in parallel)on the 160m bandswitch position (about 3450pf). On 80m its about 800pf>>

In normal designs capacitance and inductance halves as frequncy is doubled. If 160 was 3000 pF loading 80 should be 1500 and 40 meters 750pF.

Dentron (and Amp Supply) always used too little tank inductance on 160 so they had to use extra padding cap.

<<then on 40m about 400pf. Another owner of an LK500 indicated he had 4 caps on his 160 position also and that the BS did not accumulate like most other amps do. This amp has all ceramic disks, most are 12kv rated, X5R dialectric. >>

 Even a 5kV or 7,5kV cap will be enough on the plate side, and a 2-3kV good on the loading side. It's the current rating that is critical and often overlooked. Those X5R's are not good transmitting caps.
 

<<I would also like to replace the tank caps with doorknobs, but am unable to find the right dialectric in the size I need. My tank caps are 2000pf! (2-1000pf parallel).>>

I assume you mean the plate blocking caps?? Even 500pF there would be plenty large, although 1000pF total is more traditional. 2000pF is huge and totally unnecessary.

73 Tom
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I8VYE
Member

Posts: 16




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« Reply #6 on: December 23, 2005, 01:50:42 AM »

Hi all,

   in my case what i meant is that, probably, i had something not stable on output links (maybe a false contact) that, during my daughter loud crying (at full power ssb!) failed resulting in  transmitting with no load. In that case, strong out power peak with a short period of no load to the amp, damaged the disc caps. I agree, totally with w8ji, those disc caps aren't reliable as pads in tank (plate & load). So i changed them with doorknobs, rated for right voltage and current. Now they don't fail any longer even in the cases i forgot for a very short while, it sometimes can happen to me, to connect the antenna. I found the doorknobs from rfparts and/or nebraska surplus sales.

73's de i8vye/0 (joe).

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